diff --git a/lib/Slic3r.pm b/lib/Slic3r.pm
index 7bf6f4209..b345f0d7d 100644
--- a/lib/Slic3r.pm
+++ b/lib/Slic3r.pm
@@ -159,4 +159,8 @@ sub open {
     return CORE::open $$fh, $mode, encode_path($filename);
 }
 
+# this package declaration prevents an ugly fatal warning to be emitted when
+# spawning a new thread
+package GLUquadricObjPtr;
+
 1;
diff --git a/lib/Slic3r/Format/AMF/Parser.pm b/lib/Slic3r/Format/AMF/Parser.pm
index 24732dbc8..d392a8915 100644
--- a/lib/Slic3r/Format/AMF/Parser.pm
+++ b/lib/Slic3r/Format/AMF/Parser.pm
@@ -120,7 +120,7 @@ sub end_document {
         foreach my $instance (@{ $self->{_instances}{$object_id} }) {
             $self->{_model}->objects->[$new_object_id]->add_instance(
                 rotation => $instance->{rz} || 0,
-                offset   => Slic3r::Point->new($instance->{deltax} || 0, $instance->{deltay} || 0),
+                offset   => [ $instance->{deltax} || 0, $instance->{deltay} || 0 ],
             );
         }
     }
diff --git a/lib/Slic3r/GUI/Plater.pm b/lib/Slic3r/GUI/Plater.pm
index ee90576d9..24286303b 100644
--- a/lib/Slic3r/GUI/Plater.pm
+++ b/lib/Slic3r/GUI/Plater.pm
@@ -12,7 +12,7 @@ use Wx qw(:bitmap :brush :button :cursor :dialog :filedialog :font :keycode :ico
 use Wx::Event qw(EVT_BUTTON EVT_COMMAND EVT_KEY_DOWN EVT_LIST_ITEM_ACTIVATED EVT_LIST_ITEM_DESELECTED EVT_LIST_ITEM_SELECTED EVT_MOUSE_EVENTS EVT_PAINT EVT_TOOL EVT_CHOICE);
 use base 'Wx::Panel';
 
-use constant TB_LOAD            => &Wx::NewId;
+use constant TB_ADD             => &Wx::NewId;
 use constant TB_REMOVE          => &Wx::NewId;
 use constant TB_RESET           => &Wx::NewId;
 use constant TB_ARRANGE         => &Wx::NewId;
@@ -48,8 +48,8 @@ sub new {
     $self->{config} = Slic3r::Config->new_from_defaults(qw(
         bed_size print_center complete_objects extruder_clearance_radius skirts skirt_distance
     ));
+    $self->{model} = Slic3r::Model->new;
     $self->{objects} = [];
-    $self->{selected_objects} = [];
     
     $self->{canvas} = Wx::Panel->new($self, -1, wxDefaultPosition, CANVAS_SIZE, wxTAB_TRAVERSAL);
     $self->{canvas}->SetBackgroundColour(Wx::wxWHITE);
@@ -69,7 +69,7 @@ sub new {
     if (!&Wx::wxMSW) {
         Wx::ToolTip::Enable(1);
         $self->{htoolbar} = Wx::ToolBar->new($self, -1, wxDefaultPosition, wxDefaultSize, wxTB_HORIZONTAL | wxTB_TEXT | wxBORDER_SIMPLE | wxTAB_TRAVERSAL);
-        $self->{htoolbar}->AddTool(TB_LOAD, "Add…", Wx::Bitmap->new("$Slic3r::var/brick_add.png", wxBITMAP_TYPE_PNG), '');
+        $self->{htoolbar}->AddTool(TB_ADD, "Add…", Wx::Bitmap->new("$Slic3r::var/brick_add.png", wxBITMAP_TYPE_PNG), '');
         $self->{htoolbar}->AddTool(TB_REMOVE, "Delete", Wx::Bitmap->new("$Slic3r::var/brick_delete.png", wxBITMAP_TYPE_PNG), '');
         $self->{htoolbar}->AddTool(TB_RESET, "Delete All", Wx::Bitmap->new("$Slic3r::var/cross.png", wxBITMAP_TYPE_PNG), '');
         $self->{htoolbar}->AddTool(TB_ARRANGE, "Arrange", Wx::Bitmap->new("$Slic3r::var/bricks.png", wxBITMAP_TYPE_PNG), '');
@@ -159,7 +159,7 @@ sub new {
     EVT_BUTTON($self, $self->{btn_export_stl}, \&export_stl);
     
     if ($self->{htoolbar}) {
-        EVT_TOOL($self, TB_LOAD, \&load);
+        EVT_TOOL($self, TB_ADD, \&add);
         EVT_TOOL($self, TB_REMOVE, sub { $self->remove() }); # explicitly pass no argument to remove
         EVT_TOOL($self, TB_RESET, \&reset);
         EVT_TOOL($self, TB_ARRANGE, \&arrange);
@@ -173,7 +173,7 @@ sub new {
         EVT_TOOL($self, TB_VIEW, sub { $_[0]->object_preview_dialog });
         EVT_TOOL($self, TB_SETTINGS, sub { $_[0]->object_settings_dialog });
     } else {
-        EVT_BUTTON($self, $self->{btn_load}, \&load);
+        EVT_BUTTON($self, $self->{btn_add}, \&add);
         EVT_BUTTON($self, $self->{btn_remove}, sub { $self->remove() }); # explicitly pass no argument to remove
         EVT_BUTTON($self, $self->{btn_reset}, \&reset);
         EVT_BUTTON($self, $self->{btn_arrange}, \&arrange);
@@ -357,7 +357,7 @@ sub filament_presets {
     return map $_->GetSelection, @{ $self->{preset_choosers}{filament} };
 }
 
-sub load {
+sub add {
     my $self = shift;
     
     my $dir = $Slic3r::GUI::Settings->{recent}{skein_directory} || $Slic3r::GUI::Settings->{recent}{config_directory} || '';
@@ -375,7 +375,6 @@ sub load_file {
     my $self = shift;
     my ($input_file) = @_;
     
-    my $basename = basename($input_file);
     $Slic3r::GUI::Settings->{recent}{skein_directory} = dirname($input_file);
     Slic3r::GUI->save_settings;
     
@@ -384,31 +383,32 @@ sub load_file {
     
     local $SIG{__WARN__} = Slic3r::GUI::warning_catcher($self);
     my $model = Slic3r::Model->read_from_file($input_file);
-    for my $i (0 .. $#{$model->objects}) {
-        my $object = Slic3r::GUI::Plater::Object->new(
-            name                    => $basename,
-            input_file              => $input_file,
-            input_file_object_id    => $i,
-            model                   => $model,
-            model_object_idx        => $i,
-            mesh_stats              => $model->objects->[$i]->mesh_stats,  # so that we can free model
-            instances               => [
-                $model->objects->[$i]->instances
-                    ? (map $_->offset, @{$model->objects->[$i]->instances})
-                    : Slic3r::Point->new(0,0),
-            ],
-        );
-        
-        # we only consider the rotation of the first instance for now
-        $object->rotate($model->objects->[$i]->instances->[0]->rotation)
-            if $model->objects->[$i]->instances;
-        
-        push @{ $self->{objects} }, $object;
-        $self->object_loaded($#{ $self->{objects} }, no_arrange => (@{$object->instances} > 1));
-    }
+    $self->load_model_object($_) for @{$model->objects};
     
     $process_dialog->Destroy;
-    $self->statusbar->SetStatusText("Loaded $basename");
+    $self->statusbar->SetStatusText("Loaded " . basename($input_file));
+}
+
+sub load_model_object {
+    my ($self, $model_object) = @_;
+    
+    my $o = $self->{model}->add_object($model_object);
+        
+    push @{ $self->{objects} }, Slic3r::GUI::Plater::Object->new(
+        name => basename($model_object->input_file),
+    );
+    
+    my $need_arrange = 0;
+    if (!defined $model_object->instances) {
+        # if object has no defined position(s) we need to rearrange everything after loading
+        $need_arrange = 1;
+        
+        # add a default instance and center object around origin
+        $o->center_around_origin;
+        $o->add_instance(offset => [30,30]);
+    }
+    
+    $self->object_loaded($#{ $self->{objects} }, no_arrange => !$need_arrange);
 }
 
 sub object_loaded {
@@ -416,15 +416,17 @@ sub object_loaded {
     my ($obj_idx, %params) = @_;
     
     my $object = $self->{objects}[$obj_idx];
+    my $model_object = $self->{model}->objects->[$obj_idx];
     $self->{list}->InsertStringItem($obj_idx, $object->name);
     $self->{list}->SetItemFont($obj_idx, Wx::Font->new(10, wxDEFAULT, wxNORMAL, wxNORMAL))
         if $self->{list}->can('SetItemFont');  # legacy code for wxPerl < 0.9918 not supporting SetItemFont()
     
-    $self->{list}->SetItem($obj_idx, 1, $object->instances_count);
-    $self->{list}->SetItem($obj_idx, 2, ($object->scale * 100) . "%");
+    $self->{list}->SetItem($obj_idx, 1, $model_object->instances_count);
+    $self->{list}->SetItem($obj_idx, 2, ($model_object->instances->[0]->scaling_factor * 100) . "%");
     
     $self->make_thumbnail($obj_idx);
     $self->arrange unless $params{no_arrange};
+    $self->recenter;
     $self->{list}->Update;
     $self->{list}->Select($obj_idx, 1);
     $self->object_list_changed;
@@ -440,11 +442,11 @@ sub remove {
     }
     
     splice @{$self->{objects}}, $obj_idx, 1;
+    $self->{model}->delete_object($obj_idx);
     $self->{list}->DeleteItem($obj_idx);
-    
-    $self->{selected_objects} = [];
-    $self->selection_changed(0);
     $self->object_list_changed;
+    
+    $self->select_object(undef);
     $self->recenter;
     $self->{canvas}->Refresh;
 }
@@ -453,11 +455,11 @@ sub reset {
     my $self = shift;
     
     @{$self->{objects}} = ();
+    $self->{model}->delete_all_objects;
     $self->{list}->DeleteAllItems;
-    
-    $self->{selected_objects} = [];
-    $self->selection_changed(0);
     $self->object_list_changed;
+    
+    $self->select_object(undef);
     $self->{canvas}->Refresh;
 }
 
@@ -465,10 +467,14 @@ sub increase {
     my $self = shift;
     
     my ($obj_idx, $object) = $self->selected_object;
-    my $instances = $object->instances;
-    push @$instances, my $new_instance = $instances->[-1]->clone;
-    $new_instance->translate(scale(10), scale(10));
-    $self->{list}->SetItem($obj_idx, 1, $object->instances_count);
+    my $model_object = $self->{model}->objects->[$obj_idx];
+    my $last_instance = $model_object->instances->[-1];
+    $model_object->add_instance(
+        offset          => [ map 10+$_, @{$last_instance->offset} ],
+        scaling_factor  => $last_instance->scaling_factor,
+        rotation        => $last_instance->rotation,
+    );
+    $self->{list}->SetItem($obj_idx, 1, $model_object->instances_count);
     $self->arrange;
 }
 
@@ -476,9 +482,10 @@ sub decrease {
     my $self = shift;
     
     my ($obj_idx, $object) = $self->selected_object;
-    if ($object->instances_count >= 2) {
-        pop @{$object->instances};
-        $self->{list}->SetItem($obj_idx, 1, $object->instances_count);
+    my $model_object = $self->{model}->objects->[$obj_idx];
+    if ($model_object->instances_count >= 2) {
+        $model_object->delete_last_instance;
+        $self->{list}->SetItem($obj_idx, 1, $model_object->instances_count);
     } else {
         $self->remove;
     }
@@ -496,18 +503,25 @@ sub rotate {
     my ($angle) = @_;
     
     my ($obj_idx, $object) = $self->selected_object;
+    my $model_object = $self->{model}->objects->[$obj_idx];
+    my $model_instance = $model_object->instances->[0];
     
     # we need thumbnail to be computed before allowing rotation
     return if !$object->thumbnail;
     
     if (!defined $angle) {
-        $angle = Wx::GetNumberFromUser("", "Enter the rotation angle:", "Rotate", $object->rotate, -364, 364, $self);
+        $angle = Wx::GetNumberFromUser("", "Enter the rotation angle:", "Rotate", $model_instance->rotation, -364, 364, $self);
         return if !$angle || $angle == -1;
         $angle = 0 - $angle;  # rotate clockwise (be consistent with button icon)
     }
     
-    $object->rotate($object->rotate + $angle);
-    $self->selection_changed(1);  # refresh info (size etc.)
+    {
+        my $new_angle = $model_instance->rotation + $angle;
+        $_->rotation($new_angle) for @{ $model_object->instances };
+        $model_object->update_bounding_box;
+        $object->transform_thumbnail($self->{model}, $obj_idx);
+    }
+    $self->selection_changed;  # refresh info (size etc.)
     $self->recenter;
     $self->{canvas}->Refresh;
 }
@@ -516,16 +530,28 @@ sub changescale {
     my $self = shift;
     
     my ($obj_idx, $object) = $self->selected_object;
+    my $model_object = $self->{model}->objects->[$obj_idx];
+    my $model_instance = $model_object->instances->[0];
     
     # we need thumbnail to be computed before allowing scaling
     return if !$object->thumbnail;
     
     # max scale factor should be above 2540 to allow importing files exported in inches
-    my $scale = Wx::GetNumberFromUser("", "Enter the scale % for the selected object:", "Scale", $object->scale*100, 0, 100000, $self);
+    my $scale = Wx::GetNumberFromUser("", "Enter the scale % for the selected object:", "Scale", $model_instance->scaling_factor*100, 0, 100000, $self);
     return if !$scale || $scale == -1;
     
     $self->{list}->SetItem($obj_idx, 2, "$scale%");
-    $object->changescale($scale / 100);
+    $scale /= 100;  # turn percent into factor
+    {
+        my $variation = $scale / $model_instance->scaling_factor;
+        foreach my $range (@{ $model_object->layer_height_ranges }) {
+            $range->[0] *= $variation;
+            $range->[1] *= $variation;
+        }
+        $_->scaling_factor($scale) for @{ $model_object->instances };
+        $model_object->update_bounding_box;
+        $object->transform_thumbnail($self->{model}, $obj_idx);
+    }
     $self->selection_changed(1);  # refresh info (size, volume etc.)
     $self->arrange;
 }
@@ -533,19 +559,8 @@ sub changescale {
 sub arrange {
     my $self = shift;
     
-    my $total_parts = sum(map $_->instances_count, @{$self->{objects}}) or return;
-    my @size = ();
-    for my $a (X,Y) {
-        $size[$a] = max(map $_->transformed_size->[$a], @{$self->{objects}});
-    }
-    
     eval {
-        my $config = $self->skeinpanel->config;
-        my @positions = Slic3r::Geometry::arrange
-            ($total_parts, @size, @{$config->bed_size}, $config->min_object_distance, $self->skeinpanel->config);
-        
-        @$_ = @{shift @positions}
-            for map @{$_->instances}, @{$self->{objects}};
+        $self->{model}->arrange_objects($self->skeinpanel->config);
     };
     # ignore arrange warnings on purpose
     
@@ -556,54 +571,56 @@ sub arrange {
 sub split_object {
     my $self = shift;
     
-    my ($obj_idx, $current_object) = $self->selected_object;
-    my $current_copies_num = $current_object->instances_count;
-    my $model_object = $current_object->get_model_object;
+    my ($obj_idx, $current_object)  = $self->selected_object;
+    my $current_model_object        = $self->{model}->objects->[$obj_idx];
     
-    if (@{$model_object->volumes} > 1) {
+    if (@{$current_model_object->volumes} > 1) {
         Slic3r::GUI::warning_catcher($self)->("The selected object couldn't be split because it contains more than one volume/material.");
         return;
     }
     
-    my $mesh = $model_object->mesh;
-    $mesh->align_to_origin;
-    
-    $mesh->repair;
-    my @new_meshes = @{$mesh->split};
+    my @new_meshes = @{$current_model_object->volumes->[0]->mesh->split};
     if (@new_meshes == 1) {
         Slic3r::GUI::warning_catcher($self)->("The selected object couldn't be split because it already contains a single part.");
         return;
     }
-    
+
     # remove the original object before spawning the object_loaded event, otherwise 
     # we'll pass the wrong $obj_idx to it (which won't be recognized after the
     # thumbnail thread returns)
     $self->remove($obj_idx);
+    $current_object = $obj_idx = undef;
     
     # create a bogus Model object, we only need to instantiate the new Model::Object objects
     my $new_model = Slic3r::Model->new;
     
     foreach my $mesh (@new_meshes) {
-        $mesh->repair;
-        my $bb = $mesh->bounding_box;
-        my $model_object = $new_model->add_object;
-        $model_object->add_volume(mesh => $mesh);
-        my $object = Slic3r::GUI::Plater::Object->new(
-            name                    => basename($current_object->input_file),
-            input_file              => $current_object->input_file,
-            input_file_object_id    => undef,
-            model                   => $new_model,
-            model_object_idx        => $#{$new_model->objects},
-            mesh_stats              => $mesh->stats,  # so that we can free model
-            instances               => [ map $bb->min_point->pp, 1..$current_copies_num ],
+        my $model_object = $new_model->add_object(
+            input_file              => $current_model_object->input_file,
+            config                  => $current_model_object->config->clone,
+            layer_height_ranges     => $current_model_object->layer_height_ranges,  # TODO: clone this
+            material_mapping        => $current_model_object->material_mapping,     # TODO: clone this
+        );
+        $model_object->add_volume(
+            mesh        => $mesh,
+            material_id => $current_model_object->volumes->[0]->material_id,
         );
         
-        push @{ $self->{objects} }, $object;
-        $self->object_loaded($#{ $self->{objects} }, no_arrange => 1);
+        for my $instance_idx (0..$#{ $current_model_object->instances }) {
+            my $current_instance = $current_model_object->instances->[$instance_idx];
+            $model_object->add_instance(
+                offset          => [
+                    $current_instance->offset->[X] + ($instance_idx * 10),
+                    $current_instance->offset->[Y] + ($instance_idx * 10),
+                ],
+                rotation        => $current_instance->rotation,
+                scaling_factor  => $current_instance->scaling_factor,
+            );
+        }
+        # we need to center this single object around origin
+        $model_object->center_around_origin;
+        $self->load_model_object($model_object);
     }
-    
-    $self->recenter;
-    $self->{canvas}->Refresh;
 }
 
 sub export_gcode {
@@ -621,7 +638,7 @@ sub export_gcode {
     # select output file
     $self->{output_file} = $main::opt{output};
     {
-        $self->{output_file} = $self->skeinpanel->init_print->expanded_output_filepath($self->{output_file}, $self->{objects}[0]->input_file);
+        $self->{output_file} = $self->skeinpanel->init_print->expanded_output_filepath($self->{output_file}, $self->{model}->objects->[0]->input_file);
         my $dlg = Wx::FileDialog->new($self, 'Save G-code file as:', Slic3r::GUI->output_path(dirname($self->{output_file})),
             basename($self->{output_file}), &Slic3r::GUI::SkeinPanel::FILE_WILDCARDS->{gcode}, wxFD_SAVE);
         if ($dlg->ShowModal != wxID_OK) {
@@ -636,8 +653,6 @@ sub export_gcode {
     
     $self->statusbar->StartBusy;
     
-    $_->free_model_object for @{$self->{objects}};
-    
     # It looks like declaring a local $SIG{__WARN__} prevents the ugly
     # "Attempt to free unreferenced scalar" warning...
     local $SIG{__WARN__} = Slic3r::GUI::warning_catcher($self);
@@ -704,12 +719,13 @@ sub export_gcode2 {
     
     eval {
         $print->config->validate;
-        $print->add_model($self->make_model);
+        $print->add_model_object($_) for @{ $self->{model}->objects };
         $print->validate;
         
         {
             my @warnings = ();
             local $SIG{__WARN__} = sub { push @warnings, $_[0] };
+            
             my %params = (
                 output_file => $output_file,
                 status_cb   => sub { $params{progressbar}->(@_) },
@@ -764,7 +780,7 @@ sub export_stl {
     my $self = shift;
         
     my $output_file = $self->_get_export_file('STL') or return;
-    Slic3r::Format::STL->write_file($output_file, $self->make_model, binary => 1);
+    Slic3r::Format::STL->write_file($output_file, $self->{model}, binary => 1);
     $self->statusbar->SetStatusText("STL file exported to $output_file");
 }
 
@@ -772,7 +788,7 @@ sub export_amf {
     my $self = shift;
         
     my $output_file = $self->_get_export_file('AMF') or return;
-    Slic3r::Format::AMF->write_file($output_file, $self->make_model);
+    Slic3r::Format::AMF->write_file($output_file, $self->{model});
     $self->statusbar->SetStatusText("AMF file exported to $output_file");
 }
 
@@ -784,7 +800,7 @@ sub _get_export_file {
     
     my $output_file = $main::opt{output};
     {
-        $output_file = $self->skeinpanel->init_print->expanded_output_filepath($output_file, $self->{objects}[0]->input_file);
+        $output_file = $self->skeinpanel->init_print->expanded_output_filepath($output_file, $self->{model}->objects->[0]->input_file);
         $output_file =~ s/\.gcode$/$suffix/i;
         my $dlg = Wx::FileDialog->new($self, "Save $format file as:", dirname($output_file),
             basename($output_file), &Slic3r::GUI::SkeinPanel::MODEL_WILDCARD, wxFD_SAVE | wxFD_OVERWRITE_PROMPT);
@@ -798,47 +814,14 @@ sub _get_export_file {
     return $output_file;
 }
 
-sub make_model {
-    my $self = shift;
-    
-    my $model = Slic3r::Model->new;
-    foreach my $plater_object (@{$self->{objects}}) {
-        my $model_object = $plater_object->get_model_object;
-        
-        my $new_model_object = $model->add_object(
-            input_file  => $plater_object->input_file,
-            config      => $plater_object->config,
-            layer_height_ranges => $plater_object->layer_height_ranges,
-            material_mapping => $plater_object->material_mapping,
-        );
-        foreach my $volume (@{$model_object->volumes}) {
-            $new_model_object->add_volume(
-                material_id => $volume->material_id,
-                mesh        => $volume->mesh,
-            );
-            $model->set_material($volume->material_id || 0, {});
-        }
-        $new_model_object->align_to_origin;
-        $new_model_object->add_instance(
-            rotation    => $plater_object->rotate,  # around center point
-            scaling_factor => $plater_object->scale,
-            offset      => Slic3r::Point->new(@$_),
-        ) for @{$plater_object->instances};
-    }
-    
-    $model->align_to_origin;
-    return $model;
-}
-
 sub make_thumbnail {
     my $self = shift;
     my ($obj_idx) = @_;
     
-    my $object = $self->{objects}[$obj_idx];
-    $object->thumbnail_scaling_factor($self->{scaling_factor});
-    $object->thumbnail(Slic3r::ExPolygon::Collection->new);
+    my $plater_object = $self->{objects}[$obj_idx];
+    $plater_object->thumbnail(Slic3r::ExPolygon::Collection->new);
     my $cb = sub {
-        $object->make_thumbnail;
+        $plater_object->make_thumbnail($self->{model}, $obj_idx);
         
         if ($Slic3r::have_threads) {
             Wx::PostEvent($self, Wx::PlThreadEvent->new(-1, $THUMBNAIL_DONE_EVENT, shared_clone([ $obj_idx ])));
@@ -857,33 +840,38 @@ sub on_thumbnail_made {
     my $self = shift;
     my ($obj_idx) = @_;
     
-    $self->{objects}[$obj_idx]->_transform_thumbnail;
+    $self->{objects}[$obj_idx]->transform_thumbnail($self->{model}, $obj_idx);
     $self->recenter;
     $self->{canvas}->Refresh;
 }
 
+sub clean_instance_thumbnails {
+    my ($self) = @_;
+    
+    foreach my $object (@{ $self->{objects} }) {
+        @{ $object->instance_thumbnails } = ();
+    }
+}
+
+# this method gets called whenever bed is resized or the objects' bounding box changes
+# (i.e. when an object is added/removed/moved/rotated/scaled)
 sub recenter {
     my $self = shift;
     
     return unless @{$self->{objects}};
     
-    # calculate displacement needed to center the print
-    my @bbs = ();
-    foreach my $plobj (@{$self->{objects}}) {
-        my $bb = $plobj->transformed_bounding_box;  # in scaled coordinates
-        foreach my $inst (@{$plobj->instances}) {
-            push @bbs, my $inst_bb = $bb->clone;
-            $inst_bb->translate(@$inst, 0);
-        }
-    }
-    my $print_bb = Slic3r::Geometry::BoundingBox->merge(@bbs);
+    # get model bounding box in pixels
+    my $print_bb = $self->{model}->bounding_box;
+    $print_bb->scale($self->{scaling_factor});
+    
+    # get model size in pixels
     my $print_size = $print_bb->size;
     
-    # $self->{shift} contains the offset in pixels to add to object instances in order to center them
-    # it is expressed in upwards Y
+    # $self->{shift} contains the offset in pixels to add to object thumbnails
+    # in order to center them
     $self->{shift} = [
-        $self->to_pixel(-$print_bb->x_min) + ($self->{canvas}->GetSize->GetWidth  - $self->to_pixel($print_size->[X])) / 2,
-        $self->to_pixel(-$print_bb->y_min) + ($self->{canvas}->GetSize->GetHeight - $self->to_pixel($print_size->[Y])) / 2,
+        -$print_bb->x_min + ($self->{canvas}->GetSize->GetWidth  - $print_size->[X]) / 2,
+        -$print_bb->y_min + ($self->{canvas}->GetSize->GetHeight - $print_size->[Y]) / 2,
     ];
 }
 
@@ -921,7 +909,6 @@ sub _update_bed_size {
     # when the canvas is not rendered yet, its GetSize() method returns 0,0
     # scaling_factor is expressed in pixel / mm
     $self->{scaling_factor} = CANVAS_SIZE->[X] / max(@{ $self->{config}->bed_size });
-    $_->thumbnail_scaling_factor($self->{scaling_factor}) for @{ $self->{objects} };
     $self->recenter;
 }
 
@@ -971,55 +958,61 @@ sub repaint {
     
     # draw thumbnails
     $dc->SetPen(wxBLACK_PEN);
-    @{$parent->{object_previews}} = ();
+    $parent->clean_instance_thumbnails;
     for my $obj_idx (0 .. $#{$parent->{objects}}) {
         my $object = $parent->{objects}[$obj_idx];
+        my $model_object = $parent->{model}->objects->[$obj_idx];
         next unless defined $object->thumbnail;
-        for my $instance_idx (0 .. $#{$object->instances}) {
-            my $instance = $object->instances->[$instance_idx];
+        for my $instance_idx (0 .. $#{$model_object->instances}) {
+            my $instance = $model_object->instances->[$instance_idx];
+            next if !defined $object->transformed_thumbnail;
             
-            my $thumbnail = $object->transformed_thumbnail->clone;
-            $thumbnail->translate(map $_ * $parent->{scaling_factor}, @$instance);
-            $thumbnail->translate(map scale($parent->{shift}[$_]), (X,Y));
-            push @{$parent->{object_previews}}, [ $obj_idx, $instance_idx, $thumbnail ];  # $thumbnail has scaled coordinates
+            my $thumbnail = $object->transformed_thumbnail->clone;                  # in scaled coordinates
+            $thumbnail->scale(&Slic3r::SCALING_FACTOR * $parent->{scaling_factor}); # in unscaled pixels
+            $thumbnail->translate(map $_ * $parent->{scaling_factor}, @{$instance->offset});
+            $thumbnail->translate(@{$parent->{shift}});
             
-            my $drag_object = $self->{drag_object};
-            if (defined $drag_object && $obj_idx == $drag_object->[0] && $instance_idx == $drag_object->[1]) {
+            $object->instance_thumbnails->[$instance_idx] = $thumbnail;
+            
+            if (defined $self->{drag_object} && $self->{drag_object}[0] == $obj_idx && $self->{drag_object}[1] == $instance_idx) {
                 $dc->SetBrush($parent->{dragged_brush});
-            } elsif (grep { $_->[0] == $obj_idx } @{$parent->{selected_objects}}) {
+            } elsif ($object->selected) {
                 $dc->SetBrush($parent->{selected_brush});
             } else {
                 $dc->SetBrush($parent->{objects_brush});
             }
             foreach my $expolygon (@$thumbnail) {
                 my $points = $expolygon->contour->pp;
-                foreach my $point (@$points) {
-                    $point->[X] *= &Slic3r::SCALING_FACTOR;
-                    $point->[Y] *= &Slic3r::SCALING_FACTOR;
-                }
                 $dc->DrawPolygon($parent->_y($points), 0, 0);
             }
             
-            # if sequential printing is enabled and we have more than one object
-            if ($parent->{config}->complete_objects && (map @{$_->instances}, @{$parent->{objects}}) > 1) {
-                my @points = map @{$_->contour}, @{$parent->{object_previews}->[-1][2]};
-                if (@points >= 3) {
-                    my ($clearance) = @{offset([convex_hull(\@points)], scale($parent->{config}->extruder_clearance_radius / 2) * $parent->{scaling_factor}, 100, JT_ROUND)};
-                    $clearance->scale(&Slic3r::SCALING_FACTOR);
-                    $dc->SetPen($parent->{clearance_pen});
-                    $dc->SetBrush($parent->{transparent_brush});
-                    $dc->DrawPolygon($parent->_y($clearance), 0, 0);
-                }
+            if (0) {
+                # draw bounding box for debugging purposes
+                my $bb = $model_object->instance_bounding_box($instance_idx);
+                $bb->scale($parent->{scaling_factor});
+                # no need to translate by instance offset because instance_bounding_box() does that
+                $bb->translate(@{$parent->{shift}}, 0);
+                my $points = $bb->polygon->pp;
+                $dc->SetPen($parent->{clearance_pen});
+                $dc->SetBrush($parent->{transparent_brush});
+                $dc->DrawPolygon($parent->_y($points), 0, 0);
+            }
+            
+            # if sequential printing is enabled and we have more than one object, draw clearance area
+            if ($parent->{config}->complete_objects && (map @{$_->instances}, @{$parent->{model}->objects}) > 1) {
+                my ($clearance) = @{offset([$thumbnail->convex_hull], ($parent->{config}->extruder_clearance_radius / 2) * $parent->{scaling_factor}, 100, JT_ROUND)};
+                $dc->SetPen($parent->{clearance_pen});
+                $dc->SetBrush($parent->{transparent_brush});
+                $dc->DrawPolygon($parent->_y($clearance), 0, 0);
             }
         }
     }
     
     # draw skirt
-    if (@{$parent->{object_previews}} && $parent->{config}->skirts) {
-        my @points = map @{$_->contour}, map @{$_->[2]}, @{$parent->{object_previews}};
+    if (@{$parent->{objects}} && $parent->{config}->skirts) {
+        my @points = map @{$_->contour}, map @$_, map @{$_->instance_thumbnails}, @{$parent->{objects}};
         if (@points >= 3) {
-            my ($convex_hull) = @{offset([convex_hull(\@points)], scale($parent->{config}->skirt_distance) * $parent->{scaling_factor}, 1, JT_ROUND)};
-            $convex_hull->scale(&Slic3r::SCALING_FACTOR);
+            my ($convex_hull) = @{offset([convex_hull(\@points)], $parent->{config}->skirt_distance * $parent->{scaling_factor}, 100, JT_ROUND)};
             $dc->SetPen($parent->{skirt_pen});
             $dc->SetBrush($parent->{transparent_brush});
             $dc->DrawPolygon($parent->_y($convex_hull), 0, 0);
@@ -1034,20 +1027,22 @@ sub mouse_event {
     my $parent = $self->GetParent;
     
     my $point = $event->GetPosition;
-    my $pos = Slic3r::Point->new_scale(@{$parent->_y([[$point->x, $point->y]])->[0]}); #]] in scaled pixels
+    my $pos = Slic3r::Point->new(@{$parent->_y([[$point->x, $point->y]])->[0]}); # in pixels
     if ($event->ButtonDown(&Wx::wxMOUSE_BTN_LEFT)) {
-        $parent->{selected_objects} = [];
-        $parent->{list}->Select($parent->{list}->GetFirstSelected, 0);
-        $parent->selection_changed(0);
-        for my $preview (@{$parent->{object_previews}}) {
-            my ($obj_idx, $instance_idx, $thumbnail) = @$preview;
-            if (defined first { $_->contour->contains_point($pos) } @$thumbnail) {
-                $parent->{selected_objects} = [ [$obj_idx, $instance_idx] ];
-                $parent->{list}->Select($obj_idx, 1);
-                $parent->selection_changed(1);
-                my $instance = $parent->{objects}[$obj_idx]->instances->[$instance_idx];
-                $self->{drag_start_pos} = [ map $pos->[$_] - scale($parent->{shift}[$_]) - scale($parent->to_pixel($instance->[$_])), X,Y ];   # displacement between the click and the instance origin
-                $self->{drag_object} = $preview;
+        $parent->select_object(undef);
+        for my $obj_idx (0 .. $#{$parent->{objects}}) {
+            my $object = $parent->{objects}->[$obj_idx];
+            for my $instance_idx (0 .. $#{ $object->instance_thumbnails }) {
+                my $thumbnail = $object->instance_thumbnails->[$instance_idx];
+                if ($thumbnail->contains_point($pos)) {
+                    $parent->select_object($obj_idx);
+                    my $instance = $parent->{model}->objects->[$obj_idx]->instances->[$instance_idx];
+                    $self->{drag_start_pos} = [   # displacement between the click and the instance origin
+                        $pos->x - $parent->{shift}[X] - ($instance->offset->[X] * $parent->{scaling_factor}),
+                        $pos->y - $parent->{shift}[Y] - ($instance->offset->[Y] * $parent->{scaling_factor}),
+                    ];
+                    $self->{drag_object} = [ $obj_idx, $instance_idx ];
+                }
             }
         }
         $parent->Refresh;
@@ -1058,23 +1053,21 @@ sub mouse_event {
         $self->{drag_object} = undef;
         $self->SetCursor(wxSTANDARD_CURSOR);
     } elsif ($event->ButtonDClick) {
-    	$parent->object_preview_dialog if @{$parent->{selected_objects}};
+    	$parent->object_preview_dialog if $parent->selected_object;
     } elsif ($event->Dragging) {
         return if !$self->{drag_start_pos}; # concurrency problems
-        for my $preview ($self->{drag_object}) {
-            my ($obj_idx, $instance_idx, $thumbnail) = @$preview;
-            $parent->{objects}[$obj_idx]->instances->[$instance_idx] = Slic3r::Point->new(
-                map { $parent->to_units(unscale($pos->[$_] - $self->{drag_start_pos}[$_]) - $parent->{shift}[$_]) } (X,Y),
-            );
-            $parent->Refresh;
-        }
+        my ($obj_idx, $instance_idx) = @{ $self->{drag_object} };
+        my $model_object = $parent->{model}->objects->[$obj_idx];
+        $model_object->instances->[$instance_idx]->offset([
+            ($pos->[X] - $self->{drag_start_pos}[X] - $parent->{shift}[X]) / $parent->{scaling_factor},
+            ($pos->[Y] - $self->{drag_start_pos}[Y] - $parent->{shift}[Y]) / $parent->{scaling_factor},
+        ]);
+        $model_object->update_bounding_box;
+        $parent->Refresh;
     } elsif ($event->Moving) {
         my $cursor = wxSTANDARD_CURSOR;
-        for my $preview (@{$parent->{object_previews}}) {
-            if (defined first { $_->contour->contains_point($pos) } @{ $preview->[2] }) {
-                $cursor = Wx::Cursor->new(wxCURSOR_HAND);
-                last;
-            }
+        if (defined first { $_->contains_point($pos) } map @{$_->instance_thumbnails}, @{ $parent->{objects} }) {
+            $cursor = Wx::Cursor->new(wxCURSOR_HAND);
         }
         $self->SetCursor($cursor);
     }
@@ -1084,9 +1077,8 @@ sub list_item_deselected {
     my ($self, $event) = @_;
     
     if ($self->{list}->GetFirstSelected == -1) {
-        $self->{selected_objects} = [];
+        $self->select_object(undef);
         $self->{canvas}->Refresh;
-        $self->selection_changed(0);
     }
 }
 
@@ -1094,9 +1086,8 @@ sub list_item_selected {
     my ($self, $event) = @_;
     
     my $obj_idx = $event->GetIndex;
-    $self->{selected_objects} = [ grep $_->[0] == $obj_idx, @{$self->{object_previews}} ];
+    $self->select_object($obj_idx);
     $self->{canvas}->Refresh;
-    $self->selection_changed(1);
 }
 
 sub list_item_activated {
@@ -1120,7 +1111,8 @@ sub object_preview_dialog {
     }
     
     my $dlg = Slic3r::GUI::Plater::ObjectPreviewDialog->new($self,
-		object => $self->{objects}[$obj_idx],
+		object          => $self->{objects}[$obj_idx],
+		model_object    => $self->{model}->objects->[$obj_idx],
 	);
 	$dlg->ShowModal;
 }
@@ -1139,7 +1131,8 @@ sub object_settings_dialog {
     return unless $self->validate_config;
     
     my $dlg = Slic3r::GUI::Plater::ObjectSettingsDialog->new($self,
-		object => $self->{objects}[$obj_idx],
+		object          => $self->{objects}[$obj_idx],
+		model_object    => $self->{model}->objects->[$obj_idx],
 	);
 	$dlg->ShowModal;
 }
@@ -1160,7 +1153,9 @@ sub object_list_changed {
 
 sub selection_changed {
     my $self = shift;
-    my ($have_sel) = @_;
+    
+    my ($obj_idx, $object) = $self->selected_object;
+    my $have_sel = defined $obj_idx;
     
     my $method = $have_sel ? 'Enable' : 'Disable';
     $self->{"btn_$_"}->$method
@@ -1173,13 +1168,14 @@ sub selection_changed {
     
     if ($self->{object_info_size}) { # have we already loaded the info pane?
         if ($have_sel) {
-            my ($obj_idx, $object) = $self->selected_object;
-            $self->{object_info_size}->SetLabel(sprintf("%.2f x %.2f x %.2f", map unscale($_), @{$object->transformed_size}));
-            $self->{object_info_materials}->SetLabel($object->materials);
+            my $model_object = $self->{model}->objects->[$obj_idx];
+            my $model_instance = $model_object->instances->[0];
+            $self->{object_info_size}->SetLabel(sprintf("%.2f x %.2f x %.2f", @{$model_object->instance_bounding_box(0)->size}));
+            $self->{object_info_materials}->SetLabel($model_object->materials_count);
             
-            if (my $stats = $object->mesh_stats) {
-                $self->{object_info_volume}->SetLabel(sprintf('%.2f', $stats->{volume} * ($object->scale**3)));
-                $self->{object_info_facets}->SetLabel(sprintf('%d (%d shells)', $object->facets, $stats->{number_of_parts}));
+            if (my $stats = $model_object->mesh_stats) {
+                $self->{object_info_volume}->SetLabel(sprintf('%.2f', $stats->{volume} * ($model_instance->scaling_factor**3)));
+                $self->{object_info_facets}->SetLabel(sprintf('%d (%d shells)', $model_object->facets_count, $stats->{number_of_parts}));
                 if (my $errors = sum(@$stats{qw(degenerate_facets edges_fixed facets_removed facets_added facets_reversed backwards_edges)})) {
                     $self->{object_info_manifold}->SetLabel(sprintf("Auto-repaired (%d errors)", $errors));
                     $self->{object_info_manifold_warning_icon}->Show;
@@ -1205,9 +1201,24 @@ sub selection_changed {
     }
 }
 
+sub select_object {
+    my ($self, $obj_idx) = @_;
+    
+    $_->selected(0) for @{ $self->{objects} };
+    if (defined $obj_idx) {
+        $self->{objects}->[$obj_idx]->selected(1);
+        $self->{list}->Select($obj_idx, 1);
+    } else {
+        # TODO: deselect all in list
+    }
+    $self->selection_changed(1);
+}
+
 sub selected_object {
     my $self = shift;
-    my $obj_idx = $self->{selected_objects}[0] ? $self->{selected_objects}[0][0] : $self->{list}->GetFirstSelected;
+    
+    my $obj_idx = first { $self->{objects}[$_]->selected } 0..$#{ $self->{objects} };
+    return undef if !defined $obj_idx;
     return ($obj_idx, $self->{objects}[$obj_idx]),
 }
 
@@ -1276,155 +1287,47 @@ use List::Util qw(first);
 use Slic3r::Geometry qw(X Y Z MIN MAX deg2rad);
 
 has 'name'                  => (is => 'rw', required => 1);
-has 'input_file'            => (is => 'rw', required => 1);
-has 'input_file_object_id'  => (is => 'rw');  # undef means keep model object
-has 'model'                 => (is => 'rw', required => 1, trigger => \&_trigger_model_object);
-has 'model_object_idx'      => (is => 'rw', required => 1, trigger => \&_trigger_model_object);
-has 'bounding_box'          => (is => 'rw');  # scaled 3D bb of original object (aligned to origin) with no rotation or scaling
-has 'convex_hull'           => (is => 'rw');  # scaled 2D convex hull of original object (aligned to origin) with no rotation or scaling
-has 'scale'                 => (is => 'rw', default => sub { 1 }, trigger => \&_transform_thumbnail);
-has 'rotate'                => (is => 'rw', default => sub { 0 }, trigger => \&_transform_thumbnail); # around object center point
-has 'instances'             => (is => 'rw', default => sub { [] }); # upward Y axis
-has 'thumbnail'             => (is => 'rw', trigger => \&_transform_thumbnail);
+has 'thumbnail'             => (is => 'rw'); # ExPolygon::Collection in scaled model units with no transforms
 has 'transformed_thumbnail' => (is => 'rw');
-has 'thumbnail_scaling_factor' => (is => 'rw', trigger => \&_transform_thumbnail);
-has 'config'                => (is => 'rw', default => sub { Slic3r::Config->new });
-has 'layer_height_ranges'   => (is => 'rw', default => sub { [] }); # [ z_min, z_max, layer_height ]
-has 'material_mapping'      => (is => 'rw', default => sub { {} }); # { material_id => extruder_idx }
-has 'mesh_stats'            => (is => 'ro', required => 1);
-
-# statistics
-has 'facets'                => (is => 'rw');
-has 'vertices'              => (is => 'rw');
-has 'materials'             => (is => 'rw');
-has 'is_manifold'           => (is => 'rw');
-
-sub _trigger_model_object {
-    my $self = shift;
-    if ($self->model && defined $self->model_object_idx) {
-        my $model_object = $self->model->objects->[$self->model_object_idx];
-        $model_object->align_to_origin;
-	    $self->bounding_box($model_object->bounding_box);
-	    $self->bounding_box->scale(1 / &Slic3r::SCALING_FACTOR);
-	    
-    	my $mesh = $model_object->mesh;
-    	$mesh->repair;
-        $self->convex_hull($mesh->convex_hull);
-	    $self->facets($mesh->facets_count);
-	    $self->vertices(scalar @{$mesh->vertices});
-	    $self->materials($model_object->materials_count);
-	}
-}
-
-sub changescale {
-    my $self = shift;
-    my ($scale) = @_;
-    
-    my $variation = $scale / $self->scale;
-    foreach my $range (@{ $self->layer_height_ranges }) {
-        $range->[0] *= $variation;
-        $range->[1] *= $variation;
-    }
-    $self->scale($scale);
-}
-
-sub needed_repair {
-	my $self = shift;
-	
-    if ($self->get_model_object->needed_repair) {
-        warn "Warning: the input file contains manifoldness errors. "
-            . "Slic3r repaired it successfully by guessing what the correct shape should be, "
-            . "but you might still want to inspect the G-code before printing.\n";
-        $self->is_manifold(0);
-    } else {
-        $self->is_manifold(1);
-    }
-	return $self->is_manifold;
-}
-
-sub free_model_object {
-    my $self = shift;
-    
-    # only delete mesh from memory if we can retrieve it from the original file
-    return unless $self->input_file && defined $self->input_file_object_id;
-    $self->model(undef);
-    $self->model_object_idx(undef);
-}
-
-sub get_model_object {
-    my $self = shift;
-    
-    if ($self->model) {
-        return $self->model->objects->[$self->model_object_idx];
-    }
-    
-    return Slic3r::Model
-        ->read_from_file($self->input_file)
-        ->objects
-        ->[$self->input_file_object_id];
-}
-
-sub instances_count {
-    my $self = shift;
-    return scalar @{$self->instances};
-}
+has 'instance_thumbnails'   => (is => 'ro', default => sub { [] });  # array of ExPolygon::Collection objects, each one representing the actual placed thumbnail of each instance in pixel units
+has 'selected'              => (is => 'rw', default => sub { 0 });
 
 sub make_thumbnail {
-    my $self = shift;
+    my ($self, $model, $obj_idx) = @_;
     
-    my $mesh = $self->get_model_object->mesh;  # $self->model_object is already aligned to origin
-    $mesh->repair;
-    if (@{$mesh->facets} <= 5000) {
+    my $mesh = $model->objects->[$obj_idx]->raw_mesh;
+    
+    if ($mesh->facets_count <= 5000) {
         # remove polygons with area <= 1mm
         my $area_threshold = Slic3r::Geometry::scale 1;
         $self->thumbnail->append(
             grep $_->area >= $area_threshold,
-            @{ $mesh->horizontal_projection },
+            @{ $mesh->horizontal_projection },   # horizontal_projection returns scaled expolygons
         );
         $self->thumbnail->simplify(0.5);
     } else {
-        my $convex_hull = Slic3r::ExPolygon->new($self->convex_hull);
+        my $convex_hull = Slic3r::ExPolygon->new($mesh->convex_hull);
         $self->thumbnail->append($convex_hull);
     }
     
     return $self->thumbnail;
 }
 
-sub _transform_thumbnail {
-    my $self = shift;
+sub transform_thumbnail {
+    my ($self, $model, $obj_idx) = @_;
     
     return unless defined $self->thumbnail;
-    my $t = $self->_apply_transform($self->thumbnail);
-    $t->scale($self->thumbnail_scaling_factor);
+    
+    my $model_object = $model->objects->[$obj_idx];
+    my $model_instance = $model_object->instances->[0];
+    
+    # the order of these transformations MUST be the same everywhere, including
+    # in Slic3r::Print->add_model_object()
+    my $t = $self->thumbnail->clone;
+    $t->rotate(deg2rad($model_instance->rotation), Slic3r::Point->new(0,0));
+    $t->scale($model_instance->scaling_factor);
     
     $self->transformed_thumbnail($t);
 }
 
-# bounding box with applied rotation and scaling
-sub transformed_bounding_box {
-    my $self = shift;
-    
-    my $bb = Slic3r::Geometry::BoundingBox->new_from_points($self->_apply_transform($self->convex_hull));
-    $bb->extents->[Z] = $self->bounding_box->clone->extents->[Z];
-    $bb->extents->[Z][MAX] *= $self->scale;
-    return $bb;
-}
-
-sub _apply_transform {
-    my $self = shift;
-    my ($entity) = @_;    # can be anything that implements ->clone(), ->rotate() and ->scale()
-    
-    # the order of these transformations MUST be the same everywhere, including
-    # in Slic3r::Print->add_model()
-    my $result = $entity->clone;
-    $result->rotate(deg2rad($self->rotate), $self->bounding_box->center_2D);
-    $result->scale($self->scale);
-    return $result;
-}
-
-sub transformed_size {
-    my $self = shift;
-    return $self->transformed_bounding_box->size;
-}
-
 1;
diff --git a/lib/Slic3r/GUI/Plater/ObjectPreviewDialog.pm b/lib/Slic3r/GUI/Plater/ObjectPreviewDialog.pm
index 671fe430b..57edcedb4 100644
--- a/lib/Slic3r/GUI/Plater/ObjectPreviewDialog.pm
+++ b/lib/Slic3r/GUI/Plater/ObjectPreviewDialog.pm
@@ -11,10 +11,10 @@ sub new {
     my $class = shift;
     my ($parent, %params) = @_;
     my $self = $class->SUPER::new($parent, -1, $params{object}->name, wxDefaultPosition, [500,500], wxDEFAULT_DIALOG_STYLE | wxRESIZE_BORDER);
-    $self->{object} = $params{object};
+    $self->{model_object} = $params{model_object};
     
     my $sizer = Wx::BoxSizer->new(wxVERTICAL);
-    $sizer->Add(Slic3r::GUI::PreviewCanvas->new($self, $self->{object}->get_model_object), 1, wxEXPAND, 0);
+    $sizer->Add(Slic3r::GUI::PreviewCanvas->new($self, $self->{model_object}), 1, wxEXPAND, 0);
     $self->SetSizer($sizer);
     $self->SetMinSize($self->GetSize);
     
diff --git a/lib/Slic3r/GUI/Plater/ObjectSettingsDialog.pm b/lib/Slic3r/GUI/Plater/ObjectSettingsDialog.pm
index cf559f57e..30a5dadeb 100644
--- a/lib/Slic3r/GUI/Plater/ObjectSettingsDialog.pm
+++ b/lib/Slic3r/GUI/Plater/ObjectSettingsDialog.pm
@@ -11,12 +11,12 @@ sub new {
     my $class = shift;
     my ($parent, %params) = @_;
     my $self = $class->SUPER::new($parent, -1, "Settings for " . $params{object}->name, wxDefaultPosition, [500,500], wxDEFAULT_DIALOG_STYLE | wxRESIZE_BORDER);
-    $self->{object} = $params{object};
+    $self->{$_} = $params{$_} for keys %params;
     
     $self->{tabpanel} = Wx::Notebook->new($self, -1, wxDefaultPosition, wxDefaultSize, wxNB_TOP | wxTAB_TRAVERSAL);
-    $self->{tabpanel}->AddPage($self->{settings} = Slic3r::GUI::Plater::ObjectDialog::SettingsTab->new($self->{tabpanel}, object => $self->{object}), "Settings");
-    $self->{tabpanel}->AddPage($self->{layers} = Slic3r::GUI::Plater::ObjectDialog::LayersTab->new($self->{tabpanel}, object => $self->{object}), "Layers");
-    $self->{tabpanel}->AddPage($self->{materials} = Slic3r::GUI::Plater::ObjectDialog::MaterialsTab->new($self->{tabpanel}, object => $self->{object}), "Materials");
+    $self->{tabpanel}->AddPage($self->{settings} = Slic3r::GUI::Plater::ObjectDialog::SettingsTab->new($self->{tabpanel}), "Settings");
+    $self->{tabpanel}->AddPage($self->{layers} = Slic3r::GUI::Plater::ObjectDialog::LayersTab->new($self->{tabpanel}), "Layers");
+    $self->{tabpanel}->AddPage($self->{materials} = Slic3r::GUI::Plater::ObjectDialog::MaterialsTab->new($self->{tabpanel}), "Materials");
     
     my $buttons = $self->CreateStdDialogButtonSizer(wxOK);
     EVT_BUTTON($self, wxID_OK, sub {
@@ -42,17 +42,24 @@ sub new {
     return $self;
 }
 
+package Slic3r::GUI::Plater::ObjectDialog::BaseTab;
+use base 'Wx::Panel';
+
+sub model_object {
+    my ($self) = @_;
+    return $self->GetParent->GetParent->{model_object};
+}
+
 package Slic3r::GUI::Plater::ObjectDialog::SettingsTab;
 use Wx qw(:dialog :id :misc :sizer :systemsettings :button :icon);
 use Wx::Grid;
 use Wx::Event qw(EVT_BUTTON);
-use base 'Wx::Panel';
+use base 'Slic3r::GUI::Plater::ObjectDialog::BaseTab';
 
 sub new {
     my $class = shift;
     my ($parent, %params) = @_;
     my $self = $class->SUPER::new($parent, -1, wxDefaultPosition, wxDefaultSize);
-    $self->{object} = $params{object};
     
     $self->{sizer} = Wx::BoxSizer->new(wxVERTICAL);
     
@@ -79,7 +86,7 @@ sub new {
             my $idx = $choice->GetSelection;
             return if $idx == -1;  # lack of selected item, can happen on Windows
             my $opt_key = $self->{options}[$idx];
-            $self->{object}->config->apply(Slic3r::Config->new_from_defaults($opt_key));
+            $self->model_object->config->apply(Slic3r::Config->new_from_defaults($opt_key));
             $self->update_optgroup;
         });
         
@@ -105,7 +112,7 @@ sub update_optgroup {
     
     $self->{options_sizer}->Clear(1);
     
-    my $config = $self->{object}->config;
+    my $config = $self->model_object->config;
     my %categories = ();
     foreach my $opt_key (keys %$config) {
         my $category = $Slic3r::Config::Options->{$opt_key}{category};
@@ -124,7 +131,7 @@ sub update_optgroup {
                 my ($opt_key) = @{$line->{options}};  # we assume that we have one option per line
                 my $btn = Wx::BitmapButton->new($self, -1, Wx::Bitmap->new("$Slic3r::var/delete.png", wxBITMAP_TYPE_PNG));
                 EVT_BUTTON($self, $btn, sub {
-                    delete $self->{object}->config->{$opt_key};
+                    delete $self->model_object->config->{$opt_key};
                     Slic3r::GUI->CallAfter(sub { $self->update_optgroup });
                 });
                 return $btn;
@@ -141,7 +148,7 @@ sub CanClose {
     # validate options before allowing user to dismiss the dialog
     # the validate method only works on full configs so we have
     # to merge our settings with the default ones
-    my $config = Slic3r::Config->merge($self->GetParent->GetParent->GetParent->GetParent->GetParent->config, $self->{object}->config);
+    my $config = Slic3r::Config->merge($self->GetParent->GetParent->GetParent->GetParent->GetParent->config, $self->model_object->config);
     eval {
         $config->validate;
     };
@@ -153,13 +160,12 @@ package Slic3r::GUI::Plater::ObjectDialog::LayersTab;
 use Wx qw(:dialog :id :misc :sizer :systemsettings);
 use Wx::Grid;
 use Wx::Event qw(EVT_GRID_CELL_CHANGED);
-use base 'Wx::Panel';
+use base 'Slic3r::GUI::Plater::ObjectDialog::BaseTab';
 
 sub new {
     my $class = shift;
     my ($parent, %params) = @_;
     my $self = $class->SUPER::new($parent, -1, wxDefaultPosition, wxDefaultSize);
-    $self->{object} = $params{object};
     
     my $sizer = Wx::BoxSizer->new(wxVERTICAL);
     
@@ -182,7 +188,7 @@ sub new {
     $grid->SetDefaultCellAlignment(wxALIGN_CENTRE, wxALIGN_CENTRE);
     
     # load data
-    foreach my $range (@{ $self->{object}->layer_height_ranges }) {
+    foreach my $range (@{ $self->model_object->layer_height_ranges }) {
         $grid->AppendRows(1);
         my $i = $grid->GetNumberRows-1;
         $grid->SetCellValue($i, $_, $range->[$_]) for 0..2;
@@ -242,7 +248,7 @@ sub Closing {
     my $self = shift;
     
     # save ranges into the plater object
-    $self->{object}->layer_height_ranges([ $self->_get_ranges ]);
+    $self->model_object->layer_height_ranges([ $self->_get_ranges ]);
 }
 
 sub _get_ranges {
@@ -261,7 +267,7 @@ package Slic3r::GUI::Plater::ObjectDialog::MaterialsTab;
 use Wx qw(:dialog :id :misc :sizer :systemsettings :button :icon);
 use Wx::Grid;
 use Wx::Event qw(EVT_BUTTON);
-use base 'Wx::Panel';
+use base 'Slic3r::GUI::Plater::ObjectDialog::BaseTab';
 
 sub new {
     my $class = shift;
@@ -280,12 +286,12 @@ sub new {
     }
     
     # get unique materials used in this object
-    $self->{materials} = [ $self->{object}->get_model_object->unique_materials ];
+    $self->{materials} = [ $self->model_object->unique_materials ];
     
     # build an OptionsGroup
     $self->{mapping} = {
         (map { $self->{materials}[$_] => $_+1 } 0..$#{ $self->{materials} }),   # defaults
-        %{$self->{object}->material_mapping},
+        %{$self->model_object->material_mapping},
     };
     my $optgroup = Slic3r::GUI::OptionsGroup->new(
         parent      => $self,
@@ -298,7 +304,7 @@ sub new {
                 {
                     opt_key     => "material_extruder_$_",
                     type        => 'i',
-                    label       => $self->{object}->get_model_object->model->get_material_name($material_id),
+                    label       => $self->model_object->model->get_material_name($material_id),
                     min         => 1,
                     default     => $self->{mapping}{$material_id},
                     on_change   => sub { $self->{mapping}{$material_id} = $_[0] },
@@ -318,7 +324,7 @@ sub Closing {
     my $self = shift;
     
     # save mappings into the plater object
-    $self->{object}->material_mapping($self->{mapping});
+    $self->model_object->material_mapping($self->{mapping});
 }
 
 1;
diff --git a/lib/Slic3r/GUI/PreviewCanvas.pm b/lib/Slic3r/GUI/PreviewCanvas.pm
index 69626d44c..9985d882d 100644
--- a/lib/Slic3r/GUI/PreviewCanvas.pm
+++ b/lib/Slic3r/GUI/PreviewCanvas.pm
@@ -28,16 +28,15 @@ sub new {
     $self->sphi(45);
     $self->stheta(-45);
 
-    $object->align_to_origin;
-    $self->object_center($object->center);
-    $self->object_size($object->size);
+    my $bounding_box = $object->raw_mesh->bounding_box;
+    $self->object_center($bounding_box->center);
+    $self->object_size($bounding_box->size);
     
     # group mesh(es) by material
     my @materials = ();
     $self->volumes([]);
     foreach my $volume (@{$object->volumes}) {
         my $mesh = $volume->mesh;
-        $mesh->repair;
         
         my $material_id = $volume->material_id // '_';
         my $color_idx = first { $materials[$_] eq $material_id } 0..$#materials;
diff --git a/lib/Slic3r/GUI/SkeinPanel.pm b/lib/Slic3r/GUI/SkeinPanel.pm
index 2ea3dd765..efb3f3946 100644
--- a/lib/Slic3r/GUI/SkeinPanel.pm
+++ b/lib/Slic3r/GUI/SkeinPanel.pm
@@ -134,7 +134,17 @@ sub quick_slice {
         Slic3r::GUI->save_settings;
         
         my $print = $self->init_print;
-        $print->add_model(Slic3r::Model->read_from_file($input_file));
+        my $model = Slic3r::Model->read_from_file($input_file);
+        
+        if ($model->has_objects_with_no_instances) {
+            # apply a default position to all objects not having one
+            foreach my $object (@{$model->objects}) {
+                $object->add_instance(offset => [0,0]) if !defined $object->instances;
+            }
+            $model->arrange_objects($config);
+        }
+        
+        $print->add_model_object($_) for @{ $model->objects };
         $print->validate;
 
         # select output file
diff --git a/lib/Slic3r/Geometry/BoundingBox.pm b/lib/Slic3r/Geometry/BoundingBox.pm
index 27d6fd275..a05039c7c 100644
--- a/lib/Slic3r/Geometry/BoundingBox.pm
+++ b/lib/Slic3r/Geometry/BoundingBox.pm
@@ -130,6 +130,16 @@ sub min_point {
     return Slic3r::Point->new($self->extents->[X][MIN], $self->extents->[Y][MIN]);
 }
 
+sub min_point3 {
+    my $self = shift;
+    return [ map $self->extents->[$_][MIN], (X,Y,Z) ];
+}
+
+sub vector_to_origin {
+    my $self = shift;
+    return [ map -$_, @{$self->min_point3} ];
+}
+
 sub max_point {
     my $self = shift;
     return Slic3r::Point->new($self->extents->[X][MAX], $self->extents->[Y][MAX]);
diff --git a/lib/Slic3r/Model.pm b/lib/Slic3r/Model.pm
index 22126602d..b960b4926 100644
--- a/lib/Slic3r/Model.pm
+++ b/lib/Slic3r/Model.pm
@@ -6,7 +6,6 @@ use Slic3r::Geometry qw(X Y Z MIN move_points);
 
 has 'materials' => (is => 'ro', default => sub { {} });
 has 'objects'   => (is => 'ro', default => sub { [] });
-has '_bounding_box' => (is => 'rw');
 
 sub read_from_file {
     my $class = shift;
@@ -28,40 +27,61 @@ sub merge {
     my $new_model = ref($class)
         ? $class
         : $class->new;
-    foreach my $model (@models) {
-        # merge material attributes (should we rename them in case of duplicates?)
-        $new_model->set_material($_, { %{$model->materials->{$_}}, %{$model->materials->{$_} || {}} })
-            for keys %{$model->materials};
-        
-        foreach my $object (@{$model->objects}) {
-            my $new_object = $new_model->add_object(
-                input_file          => $object->input_file,
-                config              => $object->config,
-                layer_height_ranges => $object->layer_height_ranges,
-            );
-            
-            $new_object->add_volume(
-                material_id         => $_->material_id,
-                mesh                => $_->mesh->clone,
-            ) for @{$object->volumes};
-            
-            $new_object->add_instance(
-                offset              => $_->offset,
-                rotation            => $_->rotation,
-            ) for @{ $object->instances // [] };
-        }
-    }
     
+    $new_model->add_object($_) for map @{$_->objects}, @models;
     return $new_model;
 }
 
 sub add_object {
     my $self = shift;
     
-    my $object = Slic3r::Model::Object->new(model => $self, @_);
-    push @{$self->objects}, $object;
-    $self->_bounding_box(undef);
-    return $object;
+    my $new_object;
+    if (@_ == 1) {
+        # we have a Model::Object
+        my ($object) = @_;
+        
+        $new_object = $self->add_object(
+            input_file          => $object->input_file,
+            config              => $object->config,
+            layer_height_ranges => $object->layer_height_ranges,    # TODO: clone!
+            material_mapping    => $object->material_mapping,       # TODO: clone!
+        );
+        
+        foreach my $volume (@{$object->volumes}) {
+            $new_object->add_volume(
+                material_id         => $volume->material_id,
+                mesh                => $volume->mesh->clone,
+            );
+            
+            if (defined $volume->material_id) {
+                #  merge material attributes (should we rename materials in case of duplicates?)
+                $self->set_material($volume->material_id, {
+                    %{ $object->model->materials->{$volume->material_id} },
+                    %{ $self->materials->{$volume->material_id} || {} },
+                });
+            }
+        }
+        
+        $new_object->add_instance(
+            offset              => $_->offset,
+            rotation            => $_->rotation,
+            scaling_factor      => $_->scaling_factor,
+        ) for @{ $object->instances // [] };
+    } else {
+        push @{$self->objects}, $new_object = Slic3r::Model::Object->new(model => $self, @_);
+    }
+    
+    return $new_object;
+}
+
+sub delete_object {
+    my ($self, $obj_idx) = @_;
+    splice @{$self->objects}, $obj_idx, 1;
+}
+
+sub delete_all_objects {
+    my ($self) = @_;
+    @{$self->objects} = ();
 }
 
 sub set_material {
@@ -74,104 +94,111 @@ sub set_material {
     );
 }
 
-sub arrange_objects {
-    my $self = shift;
-    my ($config) = @_;
+sub duplicate_objects_grid {
+    my ($self, $grid, $distance) = @_;
     
-    # do we have objects with no position?
-    if (first { !defined $_->instances } @{$self->objects}) {
-        # we shall redefine positions for all objects
-        
-        my ($copies, @positions) = $self->_arrange(
-            config  => $config,
-            items   => $self->objects,
-        );
-        
-        # apply positions to objects
-        foreach my $object (@{$self->objects}) {
-            $object->align_to_origin;
-            
-            $object->instances([]);
+    die "Grid duplication is not supported with multiple objects\n"
+        if @{$self->objects} > 1;
+    
+    my $object = $self->objects->[0];
+    @{$object->instances} = ();
+    
+    my $size = $object->bounding_box->size;
+    for my $x_copy (1..$grid->[X]) {
+        for my $y_copy (1..$grid->[Y]) {
             $object->add_instance(
-                offset      => $_,
-                rotation    => 0,
-            ) for splice @positions, 0, $copies;
+                offset => [
+                    ($size->[X] + $distance) * ($x_copy-1),
+                    ($size->[Y] + $distance) * ($y_copy-1),
+                ],
+            );
         }
-        
-    } else {
-        # we only have objects with defined position
-        
-        # align the whole model to origin as it is
-        $self->align_to_origin;
-        
-        # arrange this model as a whole
-        my ($copies, @positions) = $self->_arrange(
-            config  => $config,
-            items   => [$self],
-        );
-        
-        # apply positions to objects by translating the current positions
-        foreach my $object (@{$self->objects}) {
-            my @old_instances = @{$object->instances};
-            $object->instances([]);
-            foreach my $instance (@old_instances) {
+    }
+}
+
+# this will append more instances to each object
+# and then automatically rearrange everything
+sub duplicate_objects {
+    my ($self, $config, $copies_num) = @_;
+    
+    foreach my $object (@{$self->objects}) {
+        my @instances = @{$object->instances};
+        foreach my $instance (@instances) {
+            ### $object->add_instance($instance->clone);  if we had clone()
+            $object->add_instance(
+                offset          => [ @{$instance->offset} ],
+                rotation        => $instance->rotation,
+                scaling_factor  => $instance->scaling_factor,
+            ) for 2..$copies_num;
+        }
+    }
+    
+    $self->arrange_objects($config);
+}
+
+# arrange objects preserving their instance count
+# but altering their instance positions
+sub arrange_objects {
+    my ($self, $config) = @_;
+    
+    # get the (transformed) size of each instance so that we take
+    # into account their different transformations when packing
+    my @instance_sizes = ();
+    foreach my $object (@{$self->objects}) {
+        push @instance_sizes, map $object->instance_bounding_box($_)->size, 0..$#{$object->instances};
+    }
+    
+    my @positions = $self->_arrange($config, \@instance_sizes);
+    
+    foreach my $object (@{$self->objects}) {
+        $_->offset(shift @positions) for @{$object->instances};
+        $object->update_bounding_box;
+    }
+}
+
+# duplicate the entire model preserving instance relative positions
+sub duplicate {
+    my ($self, $config, $copies_num) = @_;
+    
+    my $model_size = $self->bounding_box->size;
+    my @positions = $self->_arrange($config, [ map $model_size, 2..$copies_num ]);
+    
+    # note that this will leave the object count unaltered
+    
+    foreach my $object (@{$self->objects}) {
+        foreach my $instance (@{$object->instances}) {
+            foreach my $pos (@positions) {
+                ### $object->add_instance($instance->clone);  if we had clone()
                 $object->add_instance(
-                    offset      => $_,
-                    rotation    => $instance->rotation,
-                    scaling_factor => $instance->scaling_factor,
-                ) for move_points($instance->offset, @positions);
+                    offset          => [ $instance->offset->[X] + $pos->[X], $instance->offset->[Y] + $pos->[Y] ],
+                    rotation        => $instance->rotation,
+                    scaling_factor  => $instance->scaling_factor,
+                );
             }
         }
+        $object->update_bounding_box;
     }
 }
 
 sub _arrange {
-    my $self = shift;
-    my %params = @_;
+    my ($self, $config, $sizes) = @_;
     
-    my $config  = $params{config};
-    my @items   = @{$params{items}};  # can be Model or Object objects, they have to implement size()
-    
-    if ($config->duplicate_grid->[X] > 1 || $config->duplicate_grid->[Y] > 1) {
-        if (@items > 1) {
-            die "Grid duplication is not supported with multiple objects\n";
-        }
-        my @positions = ();
-        my $size = $items[0]->size;
-        my $dist = $config->duplicate_distance;
-        for my $x_copy (1..$config->duplicate_grid->[X]) {
-            for my $y_copy (1..$config->duplicate_grid->[Y]) {
-                push @positions, [
-                    ($size->[X] + $dist) * ($x_copy-1),
-                    ($size->[Y] + $dist) * ($y_copy-1),
-                ];
-            }
-        }
-        return ($config->duplicate_grid->[X] * $config->duplicate_grid->[Y]), @positions;
-    } else {
-        my $total_parts = $config->duplicate * @items;
-        my @sizes = map $_->size, @items;
-        my $partx = max(map $_->[X], @sizes);
-        my $party = max(map $_->[Y], @sizes);
-        return $config->duplicate,
-            Slic3r::Geometry::arrange
-                ($total_parts, $partx, $party, (map $_, @{$config->bed_size}),
-                $config->min_object_distance, $config);
-    }
+    my $partx = max(map $_->[X], @$sizes);
+    my $party = max(map $_->[Y], @$sizes);
+    return Slic3r::Geometry::arrange
+        (scalar(@$sizes), $partx, $party, (map $_, @{$config->bed_size}),
+        $config->min_object_distance, $config);
 }
 
-sub size {
-    my $self = shift;
-    return $self->bounding_box->size;
+sub has_objects_with_no_instances {
+    my ($self) = @_;
+    return (first { !defined $_->instances } @{$self->objects}) ? 1 : 0;
 }
 
+# this returns the bounding box of the *transformed* instances
 sub bounding_box {
     my $self = shift;
-    
-    if (!defined $self->_bounding_box) {
-        $self->_bounding_box(Slic3r::Geometry::BoundingBox->merge(map $_->bounding_box, @{$self->objects}));
-    }
-    return $self->_bounding_box->clone;
+    return Slic3r::Geometry::BoundingBox->merge(map $_->bounding_box, @{ $self->objects });
 }
 
 sub align_to_origin {
@@ -181,52 +208,33 @@ sub align_to_origin {
     # have lowest value for each axis at coordinate 0
     {
         my $bb = $self->bounding_box;
-        $self->move(map -$bb->extents->[$_][MIN], X,Y,Z);
+        $self->translate(map -$bb->extents->[$_][MIN], X,Y,Z);
     }
     
     # align all instances to 0,0 as well
     {
         my @instances = map @{$_->instances}, @{$self->objects};
         my @extents = Slic3r::Geometry::bounding_box_3D([ map $_->offset, @instances ]);
-        $_->offset->translate(-$extents[X][MIN], -$extents[Y][MIN]) for @instances;
+        foreach my $instance (@instances) {
+            $instance->offset->[X] -= $extents[X][MIN];
+            $instance->offset->[Y] -= $extents[Y][MIN];
+        }
     }
 }
 
-sub scale {
-    my $self = shift;
-    $_->scale(@_) for @{$self->objects};
-    $self->_bounding_box->scale(@_) if defined $self->_bounding_box;
-}
-
-sub move {
+sub translate {
     my $self = shift;
     my @shift = @_;
     
-    $_->move(@shift) for @{$self->objects};
-    $self->_bounding_box->translate(@shift) if defined $self->_bounding_box;
+    $_->translate(@shift) for @{$self->objects};
 }
 
 # flattens everything to a single mesh
 sub mesh {
     my $self = shift;
     
-    my @meshes = ();
-    foreach my $object (@{$self->objects}) {
-        my @instances = $object->instances ? @{$object->instances} : (undef);
-        foreach my $instance (@instances) {
-            my $mesh = $object->mesh->clone;
-            if ($instance) {
-                $mesh->align_to_origin;
-                $mesh->rotate($instance->rotation, $object->center_2D);
-                $mesh->scale($instance->scaling_factor);
-                $mesh->translate(@{$instance->offset}, 0);
-            }
-            push @meshes, $mesh;
-        }
-    }
-    
     my $mesh = Slic3r::TriangleMesh->new;
-    $mesh->merge($_) for @meshes;
+    $mesh->merge($_->mesh) for @{$self->objects};
     return $mesh;
 }
 
@@ -249,24 +257,19 @@ sub split_meshes {
         foreach my $mesh (@{$volume->mesh->split}) {
             my $new_object = $self->add_object(
                 input_file          => $object->input_file,
-                config              => $object->config,
-                layer_height_ranges => $object->layer_height_ranges,
+                config              => $object->config->clone,
+                layer_height_ranges => $object->layer_height_ranges,   # TODO: this needs to be cloned
             );
             $new_object->add_volume(
                 mesh        => $mesh,
                 material_id => $volume->material_id,
             );
             
-            # let's now align the new object to the origin and put its displacement
-            # (extents) in the instances info
-            my $bb = $mesh->bounding_box;
-            $new_object->align_to_origin;
-            
-            # add one instance per original instance applying the displacement
+            # add one instance per original instance
             $new_object->add_instance(
-                offset      => [ $_->offset->[X] + $bb->x_min, $_->offset->[Y] + $bb->y_min ],
-                rotation    => $_->rotation,
-                scaling_factor => $_->scaling_factor,
+                offset          => [ @{$_->offset} ],
+                rotation        => $_->rotation,
+                scaling_factor  => $_->scaling_factor,
             ) for @{ $object->instances // [] };
         }
     }
@@ -302,17 +305,16 @@ use Moo;
 
 use File::Basename qw(basename);
 use List::Util qw(first sum);
-use Slic3r::Geometry qw(X Y Z MIN MAX move_points move_points_3D);
-use Storable qw(dclone);
+use Slic3r::Geometry qw(X Y Z MIN MAX);
 
-has 'input_file' => (is => 'rw');
-has 'model'     => (is => 'ro', weak_ref => 1, required => 1);
-has 'volumes'   => (is => 'ro', default => sub { [] });
-has 'instances' => (is => 'rw'); # in unscaled coordinates
-has 'config'    => (is => 'rw', default => sub { Slic3r::Config->new });
-has 'layer_height_ranges' => (is => 'rw', default => sub { [] }); # [ z_min, z_max, layer_height ]
-has 'material_mapping'      => (is => 'rw', default => sub { {} }); # { material_id => extruder_idx }
-has '_bounding_box' => (is => 'rw');
+has 'input_file'            => (is => 'rw');
+has 'model'                 => (is => 'ro', weak_ref => 1, required => 1);
+has 'volumes'               => (is => 'ro', default => sub { [] });
+has 'instances'             => (is => 'rw');
+has 'config'                => (is => 'rw', default => sub { Slic3r::Config->new });
+has 'layer_height_ranges'   => (is => 'rw', default => sub { [] }); # [ z_min, z_max, layer_height ]
+has 'material_mapping'      => (is => 'rw', default => sub { {} }); # { material_id => region_idx }
+has '_bounding_box'         => (is => 'rw');
 
 sub add_volume {
     my $self = shift;
@@ -323,54 +325,78 @@ sub add_volume {
         %args,
     );
     $self->_bounding_box(undef);
-    $self->model->_bounding_box(undef);
     return $volume;
 }
 
 sub add_instance {
     my $self = shift;
+    my %params = @_;
     
     $self->instances([]) if !defined $self->instances;
-    push @{$self->instances}, Slic3r::Model::Instance->new(object => $self, @_);
-    $self->model->_bounding_box(undef);
-    return $self->instances->[-1];
+    push @{$self->instances}, my $i = Slic3r::Model::Instance->new(object => $self, %params);
+    $self->_bounding_box(undef);
+    return $i;
 }
 
-sub mesh {
+sub delete_last_instance {
+    my ($self) = @_;
+    pop @{$self->instances};
+    $self->_bounding_box(undef);
+}
+
+sub instances_count {
+    my $self = shift;
+    return scalar(@{ $self->instances // [] });
+}
+
+sub raw_mesh {
     my $self = shift;
     
     my $mesh = Slic3r::TriangleMesh->new;
-    $mesh->merge($_->mesh) for @{$self->volumes};
+    $mesh->merge($_->mesh) for @{ $self->volumes };
     return $mesh;
 }
 
-sub size {
+# flattens all volumes and instances into a single mesh
+sub mesh {
     my $self = shift;
-    return $self->bounding_box->size;
+    
+    my $mesh = $self->raw_mesh;
+    
+    my @instance_meshes = ();
+    foreach my $instance (@{ $self->instances }) {
+        my $m = $mesh->clone;
+        $instance->transform_mesh($m);
+        push @instance_meshes, $m;
+    }
+    
+    my $full_mesh = Slic3r::TriangleMesh->new;
+    $full_mesh->merge($_) for @instance_meshes;
+    return $full_mesh;
 }
 
-sub center {
-    my $self = shift;
-    return $self->bounding_box->center;
-}
-
-sub center_2D {
-    my $self = shift;
-    return $self->bounding_box->center_2D;
+sub update_bounding_box {
+    my ($self) = @_;
+    $self->_bounding_box($self->mesh->bounding_box);
 }
 
+# this returns the bounding box of the *transformed* instances
 sub bounding_box {
     my $self = shift;
     
-    if (!defined $self->_bounding_box) {
-        my @meshes = map $_->mesh, @{$self->volumes};
-        my $bounding_box = Slic3r::Geometry::BoundingBox->new_from_bb((shift @meshes)->bb3);
-        $bounding_box->merge(Slic3r::Geometry::BoundingBox->new_from_bb($_->bb3)) for @meshes;
-        $self->_bounding_box($bounding_box);
-    }
+    $self->update_bounding_box if !defined $self->_bounding_box;
     return $self->_bounding_box->clone;
 }
 
+# this returns the bounding box of the *transformed* given instance
+sub instance_bounding_box {
+    my ($self, $instance_idx) = @_;
+    
+    my $mesh = $self->raw_mesh;
+    $self->instances->[$instance_idx]->transform_mesh($mesh);
+    return $mesh->bounding_box;
+}
+
 sub align_to_origin {
     my $self = shift;
     
@@ -378,11 +404,38 @@ sub align_to_origin {
     # have lowest value for each axis at coordinate 0
     my $bb = $self->bounding_box;
     my @shift = map -$bb->extents->[$_][MIN], X,Y,Z;
-    $self->move(@shift);
+    $self->translate(@shift);
     return @shift;
 }
 
-sub move {
+sub center_around_origin {
+    my $self = shift;
+    
+    # calculate the displacements needed to 
+    # center this object around the origin
+    my $bb = $self->raw_mesh->bounding_box;
+    
+    # first align to origin on XYZ
+    my @shift = @{ $bb->vector_to_origin };
+    
+    # then center it on XY
+    my $size = $bb->size;
+    $shift[$_] -= $size->[$_]/2 for X,Y;
+    
+    $self->translate(@shift);
+    
+    if (defined $self->instances) {
+        foreach my $instance (@{ $self->instances }) {
+            $instance->offset->[X] -= $shift[X];
+            $instance->offset->[Y] -= $shift[Y];
+        }
+        $self->update_bounding_box;
+    }
+    
+    return @shift;
+}
+
+sub translate {
     my $self = shift;
     my @shift = @_;
     
@@ -390,24 +443,6 @@ sub move {
     $self->_bounding_box->translate(@shift) if defined $self->_bounding_box;
 }
 
-sub scale {
-    my $self = shift;
-    my ($factor) = @_;
-    return if $factor == 1;
-    
-    $_->mesh->scale($factor) for @{$self->volumes};
-    $self->_bounding_box->scale($factor) if defined $self->_bounding_box;
-}
-
-sub rotate {
-    my $self = shift;
-    my ($deg) = @_;
-    return if $deg == 0;
-    
-    $_->mesh->rotate($deg, Slic3r::Point->(0,0)) for @{$self->volumes};
-    $self->_bounding_box(undef);
-}
-
 sub materials_count {
     my $self = shift;
     
@@ -425,7 +460,7 @@ sub unique_materials {
 
 sub facets_count {
     my $self = shift;
-    return sum(map $_->facets_count, @{$self->volumes});
+    return sum(map $_->mesh->facets_count, @{$self->volumes});
 }
 
 sub needed_repair {
@@ -465,21 +500,27 @@ sub print_info {
     }
 }
 
-sub clone { dclone($_[0]) }
-
 package Slic3r::Model::Volume;
 use Moo;
 
-has 'object'        => (is => 'ro', weak_ref => 1, required => 1);
-has 'material_id'   => (is => 'rw');
-has 'mesh'          => (is => 'rw', required => 1);
+has 'object'            => (is => 'ro', weak_ref => 1, required => 1);
+has 'material_id'       => (is => 'rw');
+has 'mesh'              => (is => 'rw', required => 1);
 
 package Slic3r::Model::Instance;
 use Moo;
 
-has 'object'    => (is => 'ro', weak_ref => 1, required => 1);
-has 'rotation'  => (is => 'rw', default => sub { 0 });  # around mesh center point
-has 'scaling_factor' => (is => 'rw', default => sub { 1 });
-has 'offset'    => (is => 'rw');  # must be Slic3r::Point object in scaled coordinates
+has 'object'            => (is => 'ro', weak_ref => 1, required => 1);
+has 'rotation'          => (is => 'rw', default => sub { 0 });  # around mesh center point
+has 'scaling_factor'    => (is => 'rw', default => sub { 1 });
+has 'offset'            => (is => 'rw');  # must be arrayref in *unscaled* coordinates
+
+sub transform_mesh {
+    my ($self, $mesh) = @_;
+    
+    $mesh->rotate($self->rotation, Slic3r::Point->new(0,0));   # rotate around mesh origin
+    $mesh->scale($self->scaling_factor);                       # scale around mesh origin
+    $mesh->translate(@{$self->offset}, 0);
+}
 
 1;
diff --git a/lib/Slic3r/Print.pm b/lib/Slic3r/Print.pm
index e79585b1a..bfb7eaf1a 100644
--- a/lib/Slic3r/Print.pm
+++ b/lib/Slic3r/Print.pm
@@ -77,89 +77,76 @@ sub _build_has_support_material {
 
 # caller is responsible for supplying models whose objects don't collide
 # and have explicit instance positions
-sub add_model {
+sub add_model_object {
     my $self = shift;
-    my ($model) = @_;
+    my ($object) = @_;
     
-    # optimization: if avoid_crossing_perimeters is enabled, split
-    # this mesh into distinct objects so that we reduce the complexity
-    # of the graphs 
-    # -- Disabling this one because there are too many legit objects having nested shells
-    # -- It also caused a bug where plater rotation was applied to each single object by the
-    # -- code below (thus around its own center), instead of being applied to the whole 
-    # -- thing before the split.
-    ###$model->split_meshes if $Slic3r::Config->avoid_crossing_perimeters && !$Slic3r::Config->complete_objects;
+    # read the material mapping provided by the model object, if any
+    my %matmap = %{ $object->material_mapping || {} };
+    $_-- for values %matmap;  # extruders in the mapping are 1-indexed but we want 0-indexed
     
-    my %unmapped_materials = ();
-    foreach my $object (@{ $model->objects }) {
-        # we align object to origin before applying transformations
-        my @align = $object->align_to_origin;
-        
-        # extract meshes by material
-        my @meshes = ();  # by region_id
-        foreach my $volume (@{$object->volumes}) {
-            my $region_id;
-            if (defined $volume->material_id) {
-                if ($object->material_mapping) {
-                    $region_id = $object->material_mapping->{$volume->material_id} - 1
-                        if defined $object->material_mapping->{$volume->material_id};
-                }
-                $region_id //= $unmapped_materials{$volume->material_id};
-                if (!defined $region_id) {
-                    $region_id = $unmapped_materials{$volume->material_id} = scalar(keys %unmapped_materials);
-                }
+    my %meshes = ();  # region_id => TriangleMesh
+    foreach my $volume (@{$object->volumes}) {
+        my $region_id;
+        if (defined $volume->material_id) {
+            if (!exists $matmap{ $volume->material_id }) {
+                # there's no mapping between this material and a region
+                $matmap{ $volume->material_id } = scalar(@{ $self->regions });
             }
-            $region_id //= 0;
-            
-            my $mesh = $volume->mesh->clone;
-            # should the object contain multiple volumes of the same material, merge them
-            $meshes[$region_id] = $meshes[$region_id]
-                ? Slic3r::TriangleMesh->merge($meshes[$region_id], $mesh)
-                : $mesh;
-        }
-        $self->regions->[$_] //= Slic3r::Print::Region->new for 0..$#meshes;
-        
-        foreach my $mesh (grep $_, @meshes) {
-            # the order of these transformations must be the same as the one used in plater
-            # to make the object positioning consistent with the visual preview
-            
-            # we ignore the per-instance transformations currently and only 
-            # consider the first one
-            if ($object->instances && @{$object->instances}) {
-                $mesh->rotate($object->instances->[0]->rotation, $object->center_2D);
-                $mesh->scale($object->instances->[0]->scaling_factor);
-            }
-            $mesh->repair;
+            $region_id = $matmap{ $volume->material_id };
+        } else {
+            $region_id = 0;
         }
         
-        # we also align object after transformations so that we only work with positive coordinates
-        # and the assumption that bounding_box === size works
-        my $bb = Slic3r::Geometry::BoundingBox->merge(map $_->bounding_box, grep $_, @meshes);
-        my @align2 = map -$bb->extents->[$_][MIN], (X,Y,Z);
-        $_->translate(@align2) for grep $_, @meshes;
+        # instantiate region if it does not exist
+        $self->regions->[$region_id] //= Slic3r::Print::Region->new;
         
-        my $scaled_bb = $bb->clone;
-        $scaled_bb->scale(1 / &Slic3r::SCALING_FACTOR);
-        
-        # initialize print object
-        push @{$self->objects}, Slic3r::Print::Object->new(
-            print       => $self,
-            meshes      => [ @meshes ],
-            copies      => [
-                map Slic3r::Point->new(@$_),
-                $object->instances
-                    ? (map [ scale($_->offset->[X] - $align[X]) - $align2[X], scale($_->offset->[Y] - $align[Y]) - $align2[Y] ], @{$object->instances})
-                    : [0,0],
-            ],
-            size        => $scaled_bb->size,  # transformed size
-            input_file  => $object->input_file,
-            config_overrides    => $object->config,
-            layer_height_ranges => $object->layer_height_ranges,
+        # if a mesh is already associated to this region, append this one to it
+        $meshes{$region_id} //= Slic3r::TriangleMesh->new;
+        $meshes{$region_id}->merge($volume->mesh);
+    }
+    
+    # bounding box of the original meshes in original position in unscaled coordinates
+    my $bb1 = Slic3r::Geometry::BoundingBox->merge(map $_->bounding_box, values %meshes);
+    
+    foreach my $mesh (values %meshes) {
+        # we ignore the per-instance transformations currently and only 
+        # consider the first one
+        $object->instances->[0]->transform_mesh($mesh);
+    }
+    
+    # we align object also after transformations so that we only work with positive coordinates
+    # and the assumption that bounding_box === size works
+    my $bb2 = Slic3r::Geometry::BoundingBox->merge(map $_->bounding_box, values %meshes);
+    $_->translate(@{$bb2->vector_to_origin}) for values %meshes;
+    
+    # prepare scaled object size
+    my $scaled_bb = $bb2->clone;
+    $scaled_bb->translate(@{$bb2->vector_to_origin});  # not needed for getting size, but who knows
+    $scaled_bb->scale(1 / &Slic3r::SCALING_FACTOR);
+    
+    # prepare copies
+    my @copies = ();
+    foreach my $instance (@{ $object->instances }) {
+        push @copies, Slic3r::Point->new(
+            scale($instance->offset->[X] - $bb1->extents->[X][MIN]),
+            scale($instance->offset->[Y] - $bb1->extents->[Y][MIN]),
         );
     }
     
+    # initialize print object
+    push @{$self->objects}, Slic3r::Print::Object->new(
+        print               => $self,
+        meshes              => [ map $meshes{$_}, 0..$#{$self->regions} ],
+        copies              => [ @copies ],
+        size                => $scaled_bb->size,  # transformed size
+        input_file          => $object->input_file,
+        config_overrides    => $object->config,
+        layer_height_ranges => $object->layer_height_ranges,
+    );
+    
     if (!defined $self->extra_variables->{input_filename}) {
-        if (defined (my $input_file = $self->objects->[0]->input_file)) {
+        if (defined (my $input_file = $object->input_file)) {
             @{$self->extra_variables}{qw(input_filename input_filename_base)} = parse_filename($input_file);
         }
     }
diff --git a/lib/Slic3r/Print/Object.pm b/lib/Slic3r/Print/Object.pm
index 4d5a5e600..0942e5bfc 100644
--- a/lib/Slic3r/Print/Object.pm
+++ b/lib/Slic3r/Print/Object.pm
@@ -118,7 +118,6 @@ sub slice {
     # process facets
     for my $region_id (0 .. $#{$self->meshes}) {
         my $mesh = $self->meshes->[$region_id] // next;  # ignore undef meshes
-        $mesh->repair;
         
         {
             my $loops = $mesh->slice([ map $_->slice_z, @{$self->layers} ]);
diff --git a/lib/Slic3r/Test.pm b/lib/Slic3r/Test.pm
index 6f2b5d403..c857f3496 100644
--- a/lib/Slic3r/Test.pm
+++ b/lib/Slic3r/Test.pm
@@ -108,7 +108,7 @@ sub init_print {
     $model_name = [$model_name] if ref($model_name) ne 'ARRAY';
     for my $model (map model($_, %params), @$model_name) {
         $model->arrange_objects($config);
-        $print->add_model($model);
+        $print->add_model_object($_) for @{$model->objects};
     }
     $print->validate;
     
diff --git a/slic3r.pl b/slic3r.pl
index d573a4735..4b90d725f 100755
--- a/slic3r.pl
+++ b/slic3r.pl
@@ -12,6 +12,7 @@ use Getopt::Long qw(:config no_auto_abbrev);
 use List::Util qw(first);
 use POSIX qw(setlocale LC_NUMERIC);
 use Slic3r;
+use Slic3r::Geometry qw(X Y);
 $|++;
 
 our %opt = ();
@@ -119,9 +120,30 @@ if (@ARGV) {  # slicing from command line
         } else {
             $model = Slic3r::Model->read_from_file($input_file);
         }
-        $_->scale($config->scale) for @{$model->objects};
-        $_->rotate($config->rotate) for @{$model->objects};
-        $model->arrange_objects($config);
+        
+        my $need_arrange = $model->has_objects_with_no_instances;
+        if ($need_arrange) {
+            # apply a default position to all objects not having one
+            foreach my $object (@{$model->objects}) {
+                $object->add_instance(offset => [0,0]) if !defined $object->instances;
+            }
+        }
+        
+        # apply scaling and rotation supplied from command line if any
+        foreach my $instance (map @{$_->instances}, @{$model->objects}) {
+            $instance->scaling_factor($instance->scaling_factor * $config->scale);
+            $instance->rotation($instance->rotation + $config->rotate);
+        }
+        # TODO: --scale --rotate, --duplicate* shouldn't be stored in config
+        
+        if ($config->duplicate_grid->[X] > 1 || $config->duplicate_grid->[Y] > 1) {
+            $model->duplicate_objects_grid($config->duplicate_grid, $config->duplicate_distance);
+        } elsif ($need_arrange) {
+            $model->duplicate_objects($config, $config->duplicate);
+        } elsif ($config->duplicate > 1) {
+            # if all input objects have defined position(s) apply duplication to the whole model
+            $model->duplicate($config, $config->duplicate);
+        }
         
         if ($opt{info}) {
             $model->print_info;
@@ -129,7 +151,7 @@ if (@ARGV) {  # slicing from command line
         }
         
         my $print = Slic3r::Print->new(config => $config);
-        $print->add_model($model);
+        $print->add_model_object($_) for @{$model->objects};
         $print->validate;
         my %params = (
             output_file => $opt{output},
diff --git a/xs/src/ExPolygonCollection.cpp b/xs/src/ExPolygonCollection.cpp
index c6a22e56f..8d339d952 100644
--- a/xs/src/ExPolygonCollection.cpp
+++ b/xs/src/ExPolygonCollection.cpp
@@ -1,4 +1,5 @@
 #include "ExPolygonCollection.hpp"
+#include "Geometry.hpp"
 
 namespace Slic3r {
 
@@ -57,4 +58,13 @@ ExPolygonCollection::simplify(double tolerance)
     this->expolygons = expp;
 }
 
+void
+ExPolygonCollection::convex_hull(Polygon* hull) const
+{
+    Points pp;
+    for (ExPolygons::const_iterator it = this->expolygons.begin(); it != this->expolygons.end(); ++it)
+        pp.insert(pp.end(), it->contour.points.begin(), it->contour.points.end());
+    Slic3r::Geometry::convex_hull(pp, hull);
+}
+
 }
diff --git a/xs/src/ExPolygonCollection.hpp b/xs/src/ExPolygonCollection.hpp
index 5a5c4d8d4..09f56a162 100644
--- a/xs/src/ExPolygonCollection.hpp
+++ b/xs/src/ExPolygonCollection.hpp
@@ -16,6 +16,7 @@ class ExPolygonCollection
     void rotate(double angle, Point* center);
     bool contains_point(const Point* point) const;
     void simplify(double tolerance);
+    void convex_hull(Polygon* hull) const;
 };
 
 }
diff --git a/xs/src/Geometry.cpp b/xs/src/Geometry.cpp
index 26ceeabd6..be7848a05 100644
--- a/xs/src/Geometry.cpp
+++ b/xs/src/Geometry.cpp
@@ -2,6 +2,7 @@
 #include "clipper.hpp"
 #include <algorithm>
 #include <map>
+#include <vector>
 
 namespace Slic3r { namespace Geometry {
 
@@ -11,45 +12,33 @@ sort_points (Point a, Point b)
     return (a.x < b.x) || (a.x == b.x && a.y < b.y);
 }
 
-/* This implementation is based on Steffen Mueller's work for 
-   the Perl module Math::ConvexHull::MonotoneChain (available 
-   on CPAN under the GPL terms) */
+/* This implementation is based on Andrew's monotone chain 2D convex hull algorithm */
 void
-convex_hull(Points points, Polygon &hull)
+convex_hull(Points &points, Polygon* hull)
 {
     assert(points.size() >= 3);
     // sort input points
     std::sort(points.begin(), points.end(), sort_points);
     
-    typedef const Point* PointPtr;
-    PointPtr* out_hull = (PointPtr*)malloc(points.size()*2*sizeof(PointPtr));
-    
-    /* lower hull */
-    size_t k = 0;
-    for (Points::const_iterator it = points.begin(); it != points.end(); ++it) {
-        while (k >= 2 && it->ccw(out_hull[k-2], out_hull[k-1]) <= 0) --k;
-        Point pz = *&*it;
-        out_hull[k++] = &*it;
+    int n = points.size(), k = 0;
+    hull->points.resize(2*n);
+
+    // Build lower hull
+    for (int i = 0; i < n; i++) {
+        while (k >= 2 && points[i].ccw(hull->points[k-2], hull->points[k-1]) <= 0) k--;
+        hull->points[k++] = points[i];
     }
-    
-    /* upper hull */
-    size_t t = k+1;
-    for (Points::const_iterator it = points.end() - 2; it != points.begin(); --it) {
-        while (k >= t && it->ccw(out_hull[k-2], out_hull[k-1]) <= 0) --k;
-        out_hull[k++] = &*it;
+
+    // Build upper hull
+    for (int i = n-2, t = k+1; i >= 0; i--) {
+        while (k >= t && points[i].ccw(hull->points[k-2], hull->points[k-1]) <= 0) k--;
+        hull->points[k++] = points[i];
     }
+
+    hull->points.resize(k);
     
-    // we assume hull is empty
-    hull.points.reserve(k);
-    for (size_t i = 0; i < k; ++i) {
-        hull.points.push_back(*(out_hull[i]));
-    }
-    
-    // not sure why this happens randomly
-    if (hull.points.front().coincides_with(hull.points.back()))
-        hull.points.pop_back();
-    
-    free(out_hull);
+    assert( hull->points.front().coincides_with(hull->points.back()) );
+    hull->points.pop_back();
 }
 
 /* accepts an arrayref of points and returns a list of indices
diff --git a/xs/src/Geometry.hpp b/xs/src/Geometry.hpp
index d998b7aba..ea6cb3242 100644
--- a/xs/src/Geometry.hpp
+++ b/xs/src/Geometry.hpp
@@ -5,7 +5,7 @@
 
 namespace Slic3r { namespace Geometry {
 
-void convex_hull(Points points, Polygon &hull);
+void convex_hull(Points &points, Polygon* hull);
 void chained_path(Points &points, std::vector<Points::size_type> &retval, Point start_near);
 void chained_path(Points &points, std::vector<Points::size_type> &retval);
 template<class T> void chained_path_items(Points &points, T &items, T &retval);
diff --git a/xs/src/Point.cpp b/xs/src/Point.cpp
index 13e7d2361..db9c1417b 100644
--- a/xs/src/Point.cpp
+++ b/xs/src/Point.cpp
@@ -109,6 +109,9 @@ Point::distance_to(const Line &line) const
 /* Three points are a counter-clockwise turn if ccw > 0, clockwise if
  * ccw < 0, and collinear if ccw = 0 because ccw is a determinant that
  * gives the signed area of the triangle formed by p1, p2 and this point.
+ * In other words it is the 2D cross product of p1-p2 and p1-this, i.e.
+ * z-component of their 3D cross product.
+ * We return double because it must be big enough to hold 2*max(|coordinate|)^2
  */
 double
 Point::ccw(const Point &p1, const Point &p2) const
diff --git a/xs/src/TriangleMesh.cpp b/xs/src/TriangleMesh.cpp
index b59d67ca9..63d33b7c4 100644
--- a/xs/src/TriangleMesh.cpp
+++ b/xs/src/TriangleMesh.cpp
@@ -191,6 +191,8 @@ TriangleMesh::slice(const std::vector<double> &z, std::vector<Polygons> &layers)
         FUTURE: parallelize slice_facet() and make_loops()
     */
     
+    if (!this->repaired) this->repair();
+    
     // build a table to map a facet_idx to its three edge indices
     if (this->stl.v_shared == NULL) stl_generate_shared_vertices(&(this->stl));
     typedef std::pair<int,int>              t_edge;
@@ -603,7 +605,7 @@ TriangleMesh::horizontal_projection(ExPolygons &retval) const
 }
 
 void
-TriangleMesh::convex_hull(Polygon &hull)
+TriangleMesh::convex_hull(Polygon* hull)
 {
     if (this->stl.v_shared == NULL) stl_generate_shared_vertices(&(this->stl));
     Points pp;
diff --git a/xs/src/TriangleMesh.hpp b/xs/src/TriangleMesh.hpp
index 06ecca273..0056d02a7 100644
--- a/xs/src/TriangleMesh.hpp
+++ b/xs/src/TriangleMesh.hpp
@@ -33,7 +33,7 @@ class TriangleMesh
     TriangleMeshPtrs split() const;
     void merge(const TriangleMesh* mesh);
     void horizontal_projection(ExPolygons &retval) const;
-    void convex_hull(Polygon &hull);
+    void convex_hull(Polygon* hull);
     stl_file stl;
     bool repaired;
     
diff --git a/xs/src/myinit.h b/xs/src/myinit.h
index d1659823e..1ef522b44 100644
--- a/xs/src/myinit.h
+++ b/xs/src/myinit.h
@@ -20,6 +20,7 @@ extern "C" {
 
 #define EPSILON 1e-4
 #define SCALING_FACTOR 0.000001
+#define scale_(val) (val / SCALING_FACTOR)
 #define unscale(val) (val * SCALING_FACTOR)
 
 namespace Slic3r {}
diff --git a/xs/xsp/ExPolygonCollection.xsp b/xs/xsp/ExPolygonCollection.xsp
index ec404528b..793c30b9a 100644
--- a/xs/xsp/ExPolygonCollection.xsp
+++ b/xs/xsp/ExPolygonCollection.xsp
@@ -68,5 +68,15 @@ ExPolygonCollection::append(...)
             THIS->expolygons.push_back(expolygon);
         }
 
+Polygon*
+ExPolygonCollection::convex_hull()
+    PREINIT:
+        const char* CLASS = "Slic3r::Polygon";
+    CODE:
+        RETVAL = new Polygon ();
+        THIS->convex_hull(RETVAL);
+    OUTPUT:
+        RETVAL
+
 %}
 };
diff --git a/xs/xsp/Geometry.xsp b/xs/xsp/Geometry.xsp
index e800e4eca..e771a937b 100644
--- a/xs/xsp/Geometry.xsp
+++ b/xs/xsp/Geometry.xsp
@@ -17,7 +17,7 @@ convex_hull(points)
         const char* CLASS = "Slic3r::Polygon";
     CODE:
         RETVAL = new Polygon ();
-        Slic3r::Geometry::convex_hull(points, *RETVAL);
+        Slic3r::Geometry::convex_hull(points, RETVAL);
     OUTPUT:
         RETVAL
 
diff --git a/xs/xsp/TriangleMesh.xsp b/xs/xsp/TriangleMesh.xsp
index db98a633e..80160926d 100644
--- a/xs/xsp/TriangleMesh.xsp
+++ b/xs/xsp/TriangleMesh.xsp
@@ -167,7 +167,7 @@ TriangleMesh::convex_hull()
         const char* CLASS = "Slic3r::Polygon";
     CODE:
         RETVAL = new Polygon ();
-        THIS->convex_hull(*RETVAL);
+        THIS->convex_hull(RETVAL);
     OUTPUT:
         RETVAL